1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a secondary battery including a cap assembly having an improved structure for plugging up an electrolyte injection hole, and a plug for the electrolyte injection hole.
2. Description of the Related Art
Secondary batteries, which are distinguished from primary batteries due to their ability to be repeatedly charged and discharged, have been widely used in cellular phones, notebook computers, camcoders, and other portable electronic devices. Lithium secondary batteries having an operating voltage of 3.6 V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, popularized as a power source for various kinds of electronic equipment and nickel-hydrogen batteries, are frequently used because of their high energy density per unit of weight.
Such lithium secondary batteries generally use lithium oxide as a negative active material and a carbonaceous material as a positive active material. Lithium secondary batteries may be classified into liquid electrolyte batteries, also known as lithium ion batteries, and polymer electrolyte batteries, also known as lithium polymer batteries, according to the type of electrolyte used. Lithium secondary batteries are manufactured in various shapes, typically, in cylindrical, rectangular, or pouch forms.
FIG. 1 shows a conventional rectangular secondary battery. Referring to FIG. 1, a conventional secondary battery 10 includes a can 11, a battery unit 12 accommodated in the can 11, and a cap assembly 20 coupled to the can 11.
The battery unit 12 includes a negative plate 13, a separator 14, and a positive plate 15, which are sequentially stacked upon one another and rolled to be cylindrical. A negative tab 16 and a positive tab 17 connected with the respective negative plate 13 and the positive plate 15 are drawn out from the battery unit 12.
The cap assembly 20 includes a cap plate 21, which covers the top of the can 11, a positive port 23 insulated from the cap plate 21 via a gasket 22, an insulating plate 24 disposed underneath the cap plate 21, and a port plate 25 disposed underneath the insulating plate 24 and electrically connected to the positive port 23.
The negative tab 16 is electrically connected to the cap plate 21, and the positive tab 17 is electrically connected to the positive port 23 via the port plate 25.
The cap plate 21 includes an electrolyte injection hole 26 through which an electrolyte is injected into the can 11 and which is plugged up with a plug 27.
FIG. 2A illustrates a state before the electrolyte injection hole 26 of FIG. 1 is plugged up, and FIG. 2A illustrates a state after the electrolyte injection hole 26 is plugged up.
Referring to FIGS. 2A and 2B, the electrolyte injection hole 26 with a rounded upper edge 28 is formed in the cap plate 21. After an electrolyte is injected into the can 11 of FIG. 1, the plug 27, such as a ball, is placed on the rounded upper edge 28 of the electrolyte injection hole 26.
The ball 27 is compressed downward into the electrolyte injection hole 26 by a compression unit, such as a press 1, and laser welding is performed on the boundary of the compressed ball 27, thus forming a welded portion 29 and sealing the electrolyte injection hole 26.
However, such a structure of sealing the electrolyte injection hole 26 in the conventional secondary battery has the following problems. When the ball 17 that is seated on the rounded upper edge 28 of the electrolyte injection hole 26 formed in the cap plate 21 is pressed downward, it is highly unlikely that it takes on the shape of a truly circular flat head with a consistent flange along the rim of the electrolyte injection hole 16.
As a result, the electrolyte injected into the battery 10 externally leaks through a gap near the rim of the electrolyte injection hole 26, as indicated by arrows in FIG. 2B. The leaked electrolyte causes a serious spark during a welding process. Such poor sealing of the electrolyte injection hole 26 degrades the reliability of the battery 10.
A secondary battery to solve these problems is disclosed in Korean Patent Application No. 2002-1205 filed on Jan. 9, 2002. This secondary battery includes a multi-stepped electrolyte injection hole in a cap plate. A pin which fits the multi-stepped wall of the electrolyte injection hole is tightly fitted into the electrolyte injection hole by pressing, and then is subjected to welding.
However, the electrolyte may stay in the multi-stepped portion of the hole and leak externally when the pin is fitted into the electrolyte injection hole. The leaked electrolyte causes failure in welding, so that the electrolyte injection hole cannot be hermetically sealed.